The present invention relates to a unit switching apparatus which includes a plurality of current-system communication units and a spare-system communication unit and switches to the spare-system communication unit if a failure occurs in any one of the current-system communication units and, more particularly, to a unit switching apparatus including communication units whose operation modes are set by setting data constituted by various parameters.
In a communication system, when a failure occurs in a communication unit, it is required that the normal operation be quickly restored. For this reason, the communication system includes a spare-system communication unit as well as current-system communication units. When the occurrence of a failure is detected in one of the current-system communication units, the faulty current-system communication unit is switched to the spare-system communication unit to shorten the interval during which communication is interrupted. In general, one spare-system communication unit is provided for a plurality of current-system communication units. Some communication units are designed to set their operation modes by using various parameters. When a failure occurs in such a communication unit, and the faulty communication unit is to be switched to the spare-system communication unit, the same setting data the faulty communication unit must be set in the spare-system communication unit.
FIG. 4 schematically shows the arrangement of a conventional unit switching apparatus. This apparatus includes a plurality of current-system communication units (to be referred to as current units hereinafter) 101 and a spare-system communication unit (to be referred to as a spare unit hereinafter) 102 for performing communication in place of a faulty current unit. A plurality of communication lines 103 through which communication signals are transmitted are connected to a switching unit 104 through the current units 101 and the spare unit 102. The switching unit 104 is a switching circuit for switching between the faulty current unit 101 and the spare unit 102.
A management unit 105 is a circuit for setting operation modes by sending various setting data 106 to the current units 101 and the spare unit 102. Each current unit 101 has a failure detection circuit for detecting the occurrence of a failure and outputting an alarm signal 107. The alarm signal 107 output from each current unit 101 is input to a switching control unit 108 for controlling a switching operation for each current unit 101 and the spare unit 102. The switching control unit 108 discriminates the faulty current unit on the basis of the alarm signal 107, and outputs a switching control signal 109 indicating the occurrence of the failure and the faulty current unit to the switching unit 104. The switching control unit 108 also outputs a failure notification signal 111 to the management unit 105.
The management unit 105 recognizes the faulty current unit in accordance with the failure notification signal 111, and makes the same setting as that of the faulty unit with respect to the spare unit 102. Upon reception of the failure notification signal 111, the management unit 105 also sends, to the switching control unit 108, a switching permission signal 112 indicating the completion of the preparation for switching to the spare unit 102.
FIG. 5 shows the sequences of the respective signals exchanged between the units in the unit switching apparatus in FIG. 4. Referring to FIG. 5, the vertical lines, when viewed from the left, respectively correspond to the management unit 105, the switching control unit 108, the current unit 101, the spare unit 102, and the switching unit 104. Assume that a failure has occurred in the current unit 101 at the time point indicated by an arrow 121. In this case, the failure detection circuit (not shown) of the current unit 101 outputs the alarm signal 107 to notify the switching control unit 108 of the occurrence of the failure. The switching control unit 108 sends the failure notification signal 111 indicating the occurrence of the failure and the faulty current unit to the management unit 105.
The management unit 105 sends the same setting data 106 the faulty current unit to the spare unit 102, thereby setting the spare unit 102 in the same operation mode as that of the faulty current unit 101. After the setting for the spare unit 102 is completed, the management unit 105 sends the switching permission signal 112 to the switching control unit 108. Upon reception of this signal, the switching control unit 108 sends, to the switching unit 104, the switching control signal 109 indicating that switching from the faulty current unit 101 to the spare unit 102 is performed. As described above, when a failure occurs in the current unit 101, switching from the faulty current unit 101 to the spare unit 102 is performed after the management unit 105 sets the operation mode in the spare unit 102.
A unit switching apparatus designed to shorten the time required to switch to the spare unit in the event of a failure is disclosed in Japanese Patent Laid-Open No. 4-291828. In this apparatus, when the same operation mode is set in a plurality of current units, the same operation mode is also set in the spare unit in advance. Since the overall arrangement of the conventional apparatus disclosed in the above reference is the same as that shown in FIG. 4, the arrangement of this apparatus will be described in detail below with reference to FIG. 4.
FIG. 6 shows the sequences of the respective signals exchanged within the above conventional unit switching apparatus in which the same operation mode as that in each current unit is set in the spare unit in advance. Referring to FIG. 6, similar to FIG. 5, the vertical lines, when viewed from the left, respectively correspond to a management unit 105, a switching control unit 108, a current unit 101, a spare unit 102, and a switching unit 104. In this apparatus, in place of the switching permission signal 112 in the unit switching apparatus shown in FIG. 3, a setting data coincidence signal 131 indicating that the setting data in all the current units 101 are the same is sent from the management unit 105 to the switching control unit 108.
When the settings in the current units 101 are the same, the management unit 105 sends the same setting data each current unit 101 to the spare unit 102 to set the same operation mode in advance. When each current unit 101 is started, the management unit 105 sends, to the switching control unit 108, a setting data coincidence signal 131 indicating that the setting data in all the current units 101 are the same. Assume that a failure has occurred in the current unit 101 at the time point indicated by an arrow 132. The failure detection circuit (not shown) of the current unit 101 outputs an alarm signal 107 to notify the switching control unit 108 of the occurrence of the failure.
Since the switching control unit 108 has received the setting data coincidence signal 131 indicating that the setting in the spare unit 102 is completed, the unit 108 sends, to the switching unit 104, a switching control signal 109 indicating that switching from the faulty current unit 101 to the spare unit 102 is performed. At the same time, the switching control unit 108 sends a failure notification signal 111 indicating the occurrence of the failure and the faulty current unit to the management unit 105.
As described above, when the setting data in all the current units 101 are the same, the spare unit 102 is set in the same operation mode as that of each current unit 101. For this reason, after a failure occurs, no setting data needs to be sent from the management unit 105 to the spare unit 102. With this operation, the switching time is shortened. Assume that the setting data in all the current units 101 are not the same. In this case, after a failure occurs, the same setting data as that for the faulty current unit 101 is sent from the management unit 105 to the spare unit 102, as in the case shown in FIG. 4.
According to the conventional technique disclosed in Japanese Patent Laid-Open No. 4-291828, when the setting data in all the current units 101 are the same, switching to the first current unit 12 can be performed within a short period of time. If, however, the setting data in the current units 102 are different from each other, setting data from the management unit 105 must be written in the spare unit 102 after a failure occurs. If, therefore, the setting data in all the current units 101 are not the same, it takes time to perform switching to the spare unit 102.